Iron corrosion (4)
I believe the answer is:
rust
'iron corrosion' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rust that I've seen before include "Reddish-brown coating formed on metal by damp" , "Corrode" , "Reddish-brown coating formed on iron by moisture" , "Red or brown oxide coating on iron or steel" , "Fungal disease of plants" .)